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2 Objectives! To outline polymerization techniques and describe approaches to reducing viscosity and improving thermal control that involve dispersion of the monomer in water.! To show how dispersed systems may be stabilized from aggregation by modification of the hydrocarbon/water interface.

7 Solution Polymerization Advantages: Solvent acts as a diluent and aids in removal of heat of polymerization. Solvent reduces viscosity, making processing easier. Thermal control is easier than in the bulk. Disadvantages: Chain transfer to solvent occurs, leading to low molecular weights. Difficult to remove solvent from final form, causing degradation of bulk properties. Environmental pollution due to solvent release.

8 Suspension (Bead or Pearl) Polymerization General Description: Monomer Inorganic Stabilizer Oil-soluble Initiator Water Droplets are cm in diameter. Kinetics are the same as in the bulk. Must have very low monomer solubility in water or polymer will form in aqueous phase. Used for styrene, methyl methacrylate, vinyl chloride, vinyl acetate

9 Suspension (Bead or Pearl) Polymerization Advantages: Low viscosity due to the suspension Easy heat removal due to the high heat capacity of water Polymerization yields finely divided, stable latexes and dispersions to be used directly in coatings, paints, and adhesives. Disadvantages: Cannot be used for polymers whose glass transition temperature is less than the polymerization temperature, or else aggregation will occur. Must separate and purify polymer, or accept contaminated product.

13 Emulsion Components: Surfactant Hydrocarbon Water Surfactant (emulsifier) molecules have polar or ionic head groups and hydrocarbon tails of carbons; dodecyl sulfate ions are typical. Surfactants aggregate at the hydrocarbon/water interface and are in equilibrium with free surfactant.

14 Micelle Formation Above the critical micelle concentration (CMC), there is an equilibrium involving clusters of surfactants, which are typically spherical (20 to 100 Å in diameter) and which contain 50 to 150 molecules. The hydrocarbon core of the micelle will become swollen by monomer molecules.

15 Emulsion Components: Monomer A small portion of water-insoluble or slightly water-soluble monomer dissolves in solution. A larger monomer portion enters the interior of micelles, causing them to increase in size. The largest portion is dispersed as surfactant-stabilized monomer droplets having diameters greater than 10,000 Å; there are monomer droplets/cm 3.

16 Emulsion Components: Site of Polymerization Radicals are produced in the aqueous phase at approximately radicals/cm 3 sec. Polymerization in the aqueous phase is insignificant due to the low monomer concentration. Polymerization mainly occurs in the micelle interiors due to: high monomer concentration high surface/volume ratio presence of interface for organic monomer and water-soluble initiator During polymerization, monomer is replenished by diffusion from droplets through the solution to micelles. Surfactant is then redistributed, with more adsorbed on swollen polymer particles and less on monomer droplets

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